Antibodies specific for the idiotype(s) of anti-(T,G)-A--L antibodies of two different mouse strains (C57BL/10Sn and C3H.SW) have been obtained. The idiotype(s) of these two strains are distinct and the anti-(T,G)-A--L antibodies of the majority of individuals of each strain express identical or cross-reactive idiotype(s). Since the ability of mice to respond to (T,G)-A--L is under the control of H-2 linked Ir genes, these anti-idiotype antibodies are being used in conjunction with radioimmunoassay of the binding of (T,G)-A--L to anti-(T,G)-A--L antibodies, antigen-specific T lymphocyte proliferation to this antigen, and the in vitro primary antibody response to TNP-(T,G)-A--L to evaluate the cellular and genetic control of idiotype expression as well as the role of idiotype-bearing molecules in regulation of the immune response. T lymphocytes express anti-(T,G)-A--L idiotype(s) identical to or cross-reactive with those found on anti-(T,G)-A--L antibodies of the same strain.